The present invention relates generally to antennas of electromagnetic radiation and more particularly to broadband isolation apparatus for a dipole antenna array wherein one or more antennas are operated from different radio transmitters.
When several radio signal sources such as transceivers are operated simultaneously in close proximity, it is common to experience receiver overload, desensitization, cross talk or other annoying non-linear effects arising from cross modulation occurring in the receivers. These non-linear effects arise because the local transmitter produces an intense electromagnetic field at the nearby receiving antenna. If the receiver is shielded and has an exceptionally high sensitivity preceding the first mixer stage, then cross modulation may not be troublesome if the receiving and transmitting frequencies are well separated. If the receiver preselection is by itself insufficient to reduce the amplitude of the interferring signal, a narrow band rejection filter placed between the receiver and the antenna may eliminate or alleviate the interference. However, where many different frequencies are used, the rejection filters require adjustment each time the transmitter frequency is changed, placing an extra burden on the radio operators and adding to the overall cost of the system.
It is well known that electrical isolation between two dipole antennas fed from separate radio signal sources is significantly greater when the dipoles are mounted one above the other on the same axis as opposed to being arranged in a broadside relationship with the same relative spacing between feedpoints. However, even where the dipoles are mounted one above the other, respective transmission lines are generally connected from the radio signal sources to the dipole antennas without any regard to the fact that the physical arrangement of the feed transmission line themselves plays a significant role in determining the radiation characteristic, impedance and the electrical isolation between the dipoles.
In addition to the use of quarter-wavelength sleeve chokes, a recent attempt to overcome the above noted problems is shown and described in U.S. Pat. No. 3,879,735 entitled "Broadband Antenna Systems With Isolated Independent Radiators" , Donn V. Campbell, et al. issuing on Apr. 22, 1975, and assigned to the assignee of the present invention. The subject matter of this patent is directed to a common dipole array for at least two separate transceivers adapted to operate simultaneously consisting of a plurality of dipole antennas having minimized electromagnetic coupling therebetween by being mounted one above the other in a generally vertical plane and having respective tubular radiation element pairs substantially aligned along a common generally vertical axis. The two radio transceivers are coupled to a respective dipole by means of a separate feed line which are directed along the common vertical axis through the tubular radiating elements to the respective dipole feedpoints. The feedline for the upper dipole is selectively configured to provide a cable choke on the lower side of the upper dipole and on either side of the lower dipole as well as an isolator section between each dipole. The feedline for the lower dipole is also configured to provide a cable choke on the lower side of the lower side of the dipole and common with the choke thereat formed in the feedline for the upper dipole.